Haha, wow that is quite the brick of lead you got there.
Here are a few ideas:
Solar is easy. It won't provide much and it'll cost a lot though.
Regen is not as easy, but completely doable. Find a way to hook up the alternator while decelerating. Ideally a large alternator so you can recoup as much as possible.
Other more out there ideas that have been talked about
:
turbo driven alternator
shock absorbers that generate electricity